FAA certification requirements for a bladed turbofan engine specify that the engine demonstrate the ability to survive failure of a single fan blade at maximum permissible r.p.m., (hereinafter "blade-out condition"). Success requires containment of all high speed particles without catching fire and without failure of mounting attachments when operated for at least 15 seconds. The ideal design criteria is to limit blade loss to the single released blade. Impact loading on the containment casing and unbalanced loads transmitted to the engine structure are then at a minimum. If fan imbalance becomes too great loss of the entire fan or engine can result.
The fan radius ratio is defined as the radius of the hub, or inner wall boundary of the fan which is typically defined by the top of the fan blade platform at the airfoil leading edge, divided by the radius of the casing, or outer wall boundary of the fan which is typically defined by the blade containment at the airfoil tip leading edge. Prior fan blade design features were able to prevent secondary blade failures for engines having an inlet radius ratio (hereinafter "radius ratio") of 0.365, or larger. However, when these features were applied to an engine wherein the radius ratio was reduced to 0.33, failure of the trailing blade occurred upon impact with the released blade. Reducing the radius ratio, e.g. reducing the hub diameter and or increasing the casing diameter, resulted in a reduction in the number of fan blades from 38 in the 0.365 radius ratio engine to 34 in the 0.33 radius ratio engine due to limitations caused by the reduced circumference of the disc. The airfoil design required to achieve the necessary flow characteristics with less blades resulted in a wider chord blade and subsequently a 55 percent increase in blade weight. Significantly higher impact loads between the released blade platform and the trailing blade airfoil resulted in the loss of one half of the trailing blade on initial testing.
Increasing airfoil thickness to prevent secondary blade loss would have a significant impact on blade weight, disc sizing, radius ratio, and fan performance and is undesirable.